Since polypropylene resins are light in weight and excellent in recycling efficiency, demands for polypropylene resins as automobile parts are increasing. Specifically, polypropylene resin compositions obtained by blending ethylene thermoplastic elastomer component such as an ethylene/propylene copolymer or an ethylene/butene copolymer, and an inorganic filler such as talc with crystalline polypropylene resin are used. It is proposed to improve the moldability, mechanical and physical properties, appearance and the like by arbitrarily selecting polypropylene resins, various kinds of elastomer components and inorganic fillers according to purposes.
In recent years, there are demands for polypropylene resin compositions capable of being molded into thinner moldings in shorter molding time for the purpose of production of light weighted automobile parts more efficiently. However, there arises a problem in molding these products such that molding appearance failures, e.g., flow marks (patterns like tiger stripes appearing on the surfaces of the moldings), are liable to occur. Especially, in the parts of such design as an uncoated area is bared such as a bumper, there is a problem that the designing value as commercial products is impaired. Further, to improve a production cycle means to shorten the pressing time in injection molding, which results, in this case, in the problem of generation of rimple-like molding failure due to voids.
As improving techniques of such flow marks, the techniques disclosed in patent literatures 1, 2 and 3 are exemplified.
That is, an object of patent literature 1 is to provide a polypropylene resin composition that is hardly accompanied by generation of flow marks or specks, and excellent in appearance when molded. As the means for solving this object, a polypropylene resin composition comprising polypropylene resin (A) having a propylene homopolymer portion having intrinsic viscosity [η]AP of 1.3 dl/g or less and propylene/ethylene random copolymer portion having an intrinsic viscosity [η]AEP of 3.0 dl/g or less, and [η]BEP of from 8.0 to 15 dl/g blended in a specific proportion is disclosed. However, in patent literature 1, although a dye swell ratio is described, not only the substantial improving level of flow marks is not described, but also the moldability improvement of a propylene/ethylene block copolymer having a high rubber content is not described at all.
On the other hand, an object of patent literature 2 is to provide a polypropylene resin composition capable of revealing good appearance and excellent in moldability. As the means for solving this object, the use of a moldability modifier comprising a propylene/ethylene block copolymer having specific physical properties is disclosed. Specifically, a propylene/ethylene block copolymer having an MFR of the propylene homopolymer portion (crystalline component) of 500 g/10 min or more and an MFR of the whole of propylene/ethylene block copolymer of 100 g/10 min or more, and a die swell ratio of from 1.2 to 2.5 is disclosed.
However, good fluidity can be obtained with these techniques, but the use amount of expensive ethylene thermoplastic elastomers should be increased to compensate for impact resistance, so that there is room for improvement.
Further, in patent literature 3, an object is to provide a propylene resin composition well balanced in moldability and physical properties such as rigidity and impact strength. In the means for solving this object, ICP (component a) having an MFR of from 10 to 130 g/10 min as a whole and the weight average molecular weight of the rubber component of from 200,000 to 1,000,000 is disclosed. Another ICP (component b) different from the propylene/ethylene block copolymer having an MFR of from 0.1 to 8 g/10 min as a whole and the weight average molecular weight of the rubber component of from 300,000 to 900,000 is also disclosed. Further, a technique of blending, in addition to Components a and b, an inorganic filler (Component c), polyethylene (Component d), and a fatty acid amide or the derivative thereof (Component e) in a specific proportion is also disclosed. However, in patent literature 3, scratch resistance alone is made great account of and appearance of moldings is not taken into consideration as the object to be solved. Accordingly, patent literature 3 does not disclose or suggest at all about the relationship among the fluidity of the crystalline propylene polymer portion, the viscosity ratio of the crystalline propylene polymer portion to the ethylene/propylene random copolymer portion, and the appearance of a molding represented by flow mark characteristic.
Further, in recent years, from the viewpoint including the manufacturing costs, it has been demanded that effect can be achieved even with a little absolute amounts of these components, that the fluctuation in physical properties of moldability modifiers in addition is little considering the use of moldability modifiers by a master batch system, and that moldability modifiers have effect as the moldability modifiers irrespective of the kinds. The above patent literatures disclose nothing in connection with these new demands, and many objects are still remained unsolved at present.